The activities of six enzymes of carbohydrate metabolism were estimated in tissue samples from 99 patients with transitional cell carcinoma of the urinary bladder undergoing transurethral resection. The results were analysed according to the stage of the disease and the malignancy grade of the carcinoma. There were statistically significant differences in the activities of three enzymes between various stages and grades. The activities of phosphofructokinase (PFK), alpha-glycerolphosphate dehydrogenase (alpha-GPDH), and phosphohexose isomerase (PHI) showed significant decreases with increased stage and grade. Of the total of 64 patients with superficial disease at presentation, five developed progressive disease during the course of this investigation. The activities of the three enzymes in these patients were compared to the median values for the group of patients with superficial disease. The activity of PFK was observed to be below the median value in all the five patients whereas the activities of alpha-GPDH and PHI showed similar patterns in four of the five patients. These preliminary data indicate that, in association with established clinical parameters, the measurement of the activity of these three enzymes may prove useful in selecting patients with an increased potential for developing progressive disease.